During the metallurgical processing of steel, molten iron, commonly known as hot metal, obtained from the blast furnace is used as a raw material feed into the subsequent steelmaking process carried out in a converter. Although obtained in batch quantities from the blast furnace, the hot metal is sometimes mixed with other iron batches or treated to alter its chemistry prior to being charged into the converter. Accordingly, it is advantageous to extract a sample of the hot metal, in order to determine its chemical composition during treatment and for use in mass and energy balances of the converter process. Devices for extracting samples of hot metal for chemical analysis are well known in the art. An example of one such prior art reference is U.S. Pat. No. 3,996,803.
The chemical laboratory of a steelworks has a variety of analytical equipment to determine the elemental composition of metal samples. The most widely used process to analyze metal samples is by optical emission spectroscopy, hereinafter called OES. Due to their rapid analysis time and inherent accuracy, OES systems are the most effective systems for determining the chemical composition of a metal sample and for controlling the processing of molten metals. However, samples obtained of blast furnace hot metal containing high concentrations of carbon and silicon are typically analyzed by X-ray fluorescence spectroscopy, rather than by OES.
The choice of analytic equipment other than OES for use with hot metal samples is dictated by the metallographic structure of the extracted sample. Typically, a hot metal sampler is a low cost sampling device mounted on a carrier tube and having a side inlet for the hot metal to enter a sampling chamber that is formed by two thick metal chill plates. The sample can easily be retrieved by crushing the sandbody surrounding the sampling chamber, after withdrawal of the sampler device from the hot melt. The hot metal instantly solidifies as it enters the sampling chamber. During solidification, gas bubbles and unwanted inclusions will rise to the top of the sample, and thus the bottom side of the solidified sample is used for analysis. The rapid cooling results in a consistently chilled sample, thereby meeting all specifications for a reliable analysis with optical spectrometers.
Hot metal samplers known in the art typically provide 35 mm round coin samples with thicknesses varying from 4 mm to 12 mm, optionally with a pin of 4 mm or 6 mm for combustion analysis. It is known from the art that during cooling to a solid, the molten sample can undergo a multitude of precipitation reactions, thereby resulting in different solidification structures due to the chemical composition of the iron and the rate of cooling the liquid metal to its solidification temperature. Since the bath temperature at the time of sampling may range from 1250° C. to 1500° C., a single prior art sampling device could result in different solidification structures.
The dominant solidification structures receive their names from the appearance of the fracture surface due to the form of carbon in the as-cast metal.
Specifically, for high carbon irons, at or above the eutectic composition, carbon will precipitate out of solution during cooling in the form of graphite flakes, thereby giving rise to its appearance and name, grey iron. In iron compositions less than the eutectic composition, grey iron can still be produced when the metal contains graphite. Promoters (e.g., elements such as silicon and phosphorus), when present in appropriate compositions such as that of blast furnace iron, are used to influence solidification towards a grey iron structure. Grey iron is not suitable for analysis on an optical emission spectrometer.
Another solidification structure occurs during rapid cooling of the iron, where dissolved carbon precipitates as white or chilled iron as a result of its shiny silver appearance. White or chill iron occurs when cast iron solidifies by precipitation of the iron carbide/austenite eutectic. For a white structure to form, the dominant grey iron eutectic must be suppressed by undercooling to a temperature below the white iron eutectic. The extent of such cooling must be such that the white iron eutectic composition is nucleated and grows in preference to the grey iron eutectic. When the suppression by chilling is slightly insufficient or occurs too late, such that graphite precipitation has already begun, the metal will chill with a white structure but interspersed with graphite. This is termed mottle iron, meaning it is neither grey nor white. The characteristics of mottle iron vary depending upon the rate of early cooling and the degree of inoculation. Analysis of this type of iron structure produces inaccurate results depending upon the location point of the analysis and its proximity to the chill surface.
U.S. Pat. No. 3,406,736 describes a device where additives are used to avoid mottle. However, inoculation is a process which results from additives to the metal which are difficult to uniformly achieve in immersion sampling devices and do result in elements added to the sample that were not present in the initial hot metal.
In order to promote white solidification in a hot metal sampling device without the use of additives, high mass and/or high conductivity solid metals must be used to form the molds into which the hot metal is cast, thereby providing the necessary chill. Surprising results have been obtained from very high solidification rate sampling devices, called direct analysis (DA) samplers. These results demonstrate that a pure white iron structure can be routinely obtained from blast furnace iron containing graphite-promoting elements, cooled from as high as 1525° C., and accurately analyzed by an optical emission spectrometer.
Broadly speaking, the OES analysis procedure begins with the conductive metal sample being positioned with its analysis surface face down on a predetermined region of the stage of the OES instrument, namely an optical emission spectrometer. More particularly, the sample is positioned so as to span and close the analysis opening of the spectrometer and an anode nearly abuts the analysis surface of the sample. Once the desired positioning of the sample and proximity of the anode and analysis surface is achieved, a spark is discharged between the anode and the conductive metal sample which is electrically connected to the spectrometer stage. This connection is, in most cases, made by gravitational force in combination with a small load. The analysis opening on the optical emission spectrometer is typically around 12 mm wide. This distance avoids that a spark arcs between the anode and the instrument housing. The optical detector receives the emitted light from the excavated material of the sample surface. The spark chamber, formed in part by the space between the anode and the metal sample, is continuously purged with argon or other inert gas in order to avoid air ingress which would lead to erroneous analysis values.
In order to lay flat across the analysis opening of the spectrometer, the metal sample cannot have any extensions and the analysis surface of the metal sample must be smooth (i.e., of there can be no parts of the sample housing which break the plane of the analysis surface). The sample must span the analysis opening of the spectrometer and be of sufficient flatness to facilitate inert gas purging of the spark chamber and present a contiguous sample surface toward the anode.
Direct Analysis (DA) samplers are a newly developed type of molten metal immersion sampler which produce DA samples. DA samples do not require any kind of surface preparation before being analyzed, and thus can result in significant economic benefit both in terms of the availability of timely chemistry results as well as laboratory time savings by utilizing the OES analysis method.
U.S. Pat. No. 9,128,013 discloses a sampling device for collecting a DA type sample from a molten steel bath. The sampling device includes a sample chamber formed by at least two parts. A similar DA type sampler is known from U.S. Patent Application Publication No. 2014/318276. One end of the sample cavity of this DA type sampler is connected to the molten metal bath during immersion of the sampler via an inflow conduit, while an opposite end of the sample cavity is in communication with a coupling device. During immersion, but before the filling of the sample cavity with the molten metal, the sample cavity is purged with an inert gas to avoid early filling and oxidation of the sampled material. The inflow conduit is arranged perpendicular to the flat surface of the sample cavity. The ventilation of the sample cavity is arranged below the analysis surface of the sample cavity relative to the immersion direction.
While such conventional sampling devices may be adequate for retrieving steel samples suitable for preparation free OES analysis, it has been determined that the necessary cooling rate required to result in a pure white solidification structure from hot metal containing graphitization elements results in cracking along the surface of the resulting sample, as well as cracking which spans the thickness of the resulting sample. This is problematic because an excitation spark of the OES, when incident upon a cracked surface, will produce erroneous results. Also, the metal sample to be analyzed by OES is placed with the analysis surface downward. In the most extreme cases, cracking can result in pieces of metal dislodging from the body of the sample and falling into the OES sparking area. Robotic equipment in a typical steelworks laboratory is ill equipped to handle this type of equipment contamination.
Also, samples produced by conventional sampling devices have a diameter of at least 32 mm in a direction parallel to the spectrometer opening and a thickness of 4-12 mm in a direction perpendicular to the spectrometer opening. Such dimensions can be easily handled by pre-analysis preparation equipment that mechanically grinds the analysis surface of the metal sample to clean oxides from the surface and provide the requisite flat topography. This geometry is also convenient to robotic manipulators which advance the sample from preparation through analysis and removal to await the next sample. Robotic equipment in a typical steelworks laboratory is difficult to modify to accept radically different sample geometries.
However, the prior art sample volume is over dimensioned from the minimum volume of metal required to arrive at the minimum necessary analyzed surface area. The sample volumes of the prior art devices thus preclude rapid solidification of the molten metal sample, which is necessary to obtain an oxide free surface. As such, conventional devices cannot be reliably analyzed by OES without surface preparation. Using massive cooling plates and sampler housings to force a large volume metal sample to low temperature after retrieval becomes impractical for rapid de-molding and is uneconomical for use as immersion sampling devices.
Accordingly, it would be beneficial to provide a DA type sample for OES analysis capable of being used for sampling hot metal and forming a solidified hot metal sample without cracking. It would be beneficial to provide a DA type sampler which produces preparation free samples of hot metal that are capable of being subjected to the degree of rapid chill necessary to promote a pure white solidification structure (i.e., a structure without graphite precipitation) and which remain crack-fee, and are thus suitable for analysis by OES.
It would also be beneficial to provide a molten metal immersion device for retrieving preparation free samples from a hot metal within metallurgical vessels which is capable of quick connection to pneumatic-assisted inert gas purge apparatus and exhibits reduced pressure metal uptake. In particular, it would be beneficial to provide a molten metal immersion device for producing a molten metal sample that is easily obtained and quickly removed from the immersion device housing, de-molded from the sample chamber, and directly analyzed on the OES without additional cooling or preparation, and which is thereby cost-effective.